The present invention relates to a circuit board with a metal layer for solder bonding and an electronic circuit device using the circuit board. More specifically, the invention relates to a circuit board with a metal layer for solder bonding which keeps good solder bonding even after electronic parts are soldered many times and facilitates exchange of electronic parts and an electronic circuit device using the circuit board.
Every electronic circuit device including a large-scale electronic computer has many electronic circuit parts such as LSIs mounted on each printed circuit board or ceramic circuit board so as to form an electronic circuit. The typical mounting method for those electronic circuit parts is as follows: Firstly, a metal layer pattern which is a solder bonding part is formed on each circuit board (hereinafter abbreviated to a board). On the other hand, a metal terminal for an electrode is formed on each electronic circuit part. Then, the two are bonded by solder.
An electronic circuit part such as an LSI, which is mounted on a board, may be removed from the board and replaced with a new part, for example, so as to change the logic of the electronic circuit (hereinafter, this process is called repair). When the electronic circuit part is to be removed from the board, the solder bonding part of the electronic circuit part is heated and melted. In this case, a part of the metal layer constituting the bonding part of the board is removed from the board when the part is replaced. The reason is that the solder bonding is realized by alloying a part of the metal constituting the solder alloy and a part of the metal layer and a part of the alloy or the entire alloy is removed when the electronic circuit part is removed by heating and melting the solder bonding part. As an example of the bonding system which is regularly used, the bonding is realized by forming Sn--Ni alloy principally when Sn--Pb solder is used and a Ni layer is used as a metal layer. When the electronic part is removed, a part of this alloy is removed simultaneously. When a new part is to be mounted, a new metal layer at the same bounding part is consumed so as to bond solder.
To improve the production yield of electronic circuit devices, repairs are necessary. However, the metal layer is consumed by repeating repair and may be lost finally. In this state, normal bonding cannot be realized and the reliability of the solder bonding part is extremely reduced or solder bonding itself is impossible. To prevent it, it is most general conventionally to form metalization which is expected beforehand to be consumed by an allowable number of times of repairs by plating.
To satisfy recent demands for highly densed and refined pattern electronic parts, a clean forming method for metal layers of electronic parts, and highly reliable part bonding portions, there are increasing requests for forming a solder bonding metal layer with a thickness of at most 1 micron on a board by sputtering or vapor deposition in place of plating. In this case, increasing the metal layer thickness by sputtering or vapor deposition so as to realize many times of repairs is not appropriate because the residual stress of such a layer is extremely high compared with the residual stress of a layer formed by plating and the formed metal layer is cracked by the residual stress of the layer. Therefore, conventionally, a barrier (underlayer) of metal which is slow in reaction for alloying with solder or is not alloyed with solder is formed on a board by sputtering or vapor deposition and a metal layer which is easily alloyed with solder is formed on it.
Many arts relating to the above arts have been reported. One of the latest arts, for example, is indicated in 40th ECTC Proceedings, pp 408-411 (1990), "Production of MCP Carriers".
However, the method for forming metalization corresponding to the estimated number of repairs by plating beforehand among the above prior arts cannot satisfy the request for forming a solder bonding metal layer with a thickness of at most 1 micron so as to satisfy demands for highly densed and refined pattern electronic parts, a clean forming method for metal layers of electronic parts, and highly reliable part bonding portions. It is natural that the method cannot correspond to many times of repairs exceeding the estimated number of times.
The method that a metal layer which is slow in reaction for alloying with solder is formed as a barrier by sputtering or vapor deposition and a metal layer which is easily alloyed with solder is formed on it imposes a problem that the metal layer is lost finally after many times of repairs and normal bonding cannot be realized.
Furthermore, the method that metal which does not react on solder is used as a barrier and a metal layer which is easily alloyed with solder is formed on it cannot realize normal bonding after all because when the upper metal layer is lost, the metal of the barrier is not alloyed with solder.